Conventional three phase pulse width modulating (PWM) inverters include an inductor between the power switches (such as IGBTs) and the connection to a utility. A PWM inverter produces rectangular wave-shaped output voltages having high distortions, especially at frequencies that are plus or minus the modulated fundamental frequency away from the switching frequency.
For applications such as active power filters or uninterruptable power supplies, the voltage and current distortion must be minimized. In particular, when connecting an inverter to a utility grid, for example, high frequency current distortions must be suppressed to prevent the hazard of machinery destruction.
One approach to distortion reduction is described by S. Bernard et al., "Active Filter Design and Specification for Control of Harmonics in Industrial and Commercial Facilities," Power Quality Solutions, September 1995 Proceedings, pp. 197-209. Bernard et al. insert, between the PWM inverter and the load, an interface filter comprising an L(inductor)-C(capacitor)-L(inductor) filter. The inductors are coupled together with one inductor coupled to the PWM inverter and the other inductor coupled to the load. The capacitor is coupled between a coupling junction of the two inductors and a neutral. The harmonic current to be canceled shows up as reactive power, and a decrease in the harmonic voltage distortion occurs because the harmonic currents flowing through the source impedance are reduced. The inductor coupled to the inverter provides a large isolation inductance to convert the voltage signal created by the inverter to a current signal for canceling harmonics, the capacitor provides signal smoothing, and the inductor coupled to the load provides isolation for high frequency components. The technique of Bernard et al., however, does not compensate for stray inductances or for non-linear loads that may excite the filter externally.